In wireless communication such as mobile communication or fixed communication, a transmission signal is subjected to various interferences by the influence of reflection waves due to obstacles in a propagation channel, and the like and communication quality is degraded by fading in which a level of a reception signal is changed.
For example, the influence of multipath fading (multipath interference) is reduced by adding a guard interval (GI) (length) based on a cyclic prefix (CP) to a transmission signal transmitted by a transmission apparatus in multicarrier transmission such as orthogonal frequency division multiplexing (OFDM), orthogonal frequency division multiple access (OFDMA), or multi carrier-code division multiplexing (MC-CDM).
In a reception apparatus, an amplitude or phase change of a propagation channel is estimated by inserting a signal known between the transmission apparatus and the reception apparatus as a pilot signal into part of a transmission signal in order to compensate for an amplitude or phase change of the transmission signal due to multipath fading or the like. It is preferable to estimate the propagation channel with high accuracy.
In particular, it is preferable to follow an amplitude or phase change of a transmission signal in a frequency direction and a time direction in a broadband transmission or high-speed mobile environment. As a method of estimating a time change and a frequency change, there is a method of scattering and arranging pilot signals in the frequency direction and the time direction. For example, a method using scattered pilot symbols is shown in Non-Patent Document 1.
An example of the method using the scattered and arranged pilot symbols (hereinafter referred to as “scattered pilot symbols”) shown in Non-Patent Document 1 will be described using FIG. 52.
FIG. 52 shows a frame constituted by 8 subcarriers and 12 OFDM symbols. In FIG. 52, the horizontal axis represents time, the vertical axis represents frequency, one row of the frequency (vertical axis) direction represents a subcarrier, and one column of the time (horizontal axis) direction represents an OFDM symbol. In the frame, a pilot symbol is arranged every fourth subcarrier in the frequency direction and every other OFDM symbol in the time direction. Furthermore, in terms of an OFDM symbol including the pilot symbol, the pilot symbol is shifted in the frequency direction every OFDM symbol. Thereby, it is possible to follow the estimation of a time change and a frequency change in an amplitude and a phase by the scattered pilot symbol.
On the other hand, an example in which an arrival wave exceeding a GI exists in multicarrier transmission of OFDM or the like and serves as a factor that degrades communication quality will be described using FIGS. 53 and 54. FIG. 53 shows the case where a channel impulse response value of a 12-wave multipath model is shown, first 4 waves of a reception signal are within a GI, and the other 8 waves exceed the GI. If there is a delay wave exceeding the GI as described above, inter-symbol interference (ISI) is caused by the head of a GI added to a previous OFDM symbol, that is, a data interval of the previous OFDM symbol, which enters an interval of a fast Fourier transform (FFT) to be performed to demodulate a received signal, as shown in FIG. 54. The ISI becomes a factor that degrades the accuracy of propagation channel estimation and degrades communication quality.
As a method of eliminating the influence by the ISI, for example, a GI of each OFDM symbol longer than a normal GI in a subframe (frame or slot) is shown in Non-Patent Document 1.    Non-Patent Document 1: “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (EUTRA); Physical Channels and Modulation (Release 8)” 3GPP TS 36.211 V8.3.0 (2008-05).